Amplifier



1.; JONES- Oct. 28, 1930.

AMPLIFIER Filed 001:. 5, 1929 3 Sheets-Sheet l INVENTOR Lesfer L. Jones ATTORNEYS Oct. 28, 1930.

1.. JONES V AMPLIFIER Filed 0013. 5, 1929 3 Sheets-Sheet 5 INVENTOR Paul-.30

kUtuUIO Lesfer L. Jones ATTORNEYS Patented Oct. 28, 1930 PATENT OFFICE LESTER JONES, OF OBADELL, NEW JERSEY AMPLIFIER I Application filed October 5, 1929, Serial No. 397,683.

This invention relates to amplifiers, and more particularly to vacuum tube amplifiers of the untuned or automatically tuned type. In my Patent No. 1,673,287 issued June 12, 1928, I have'explained how an audion or electron emission tube amplifierstage having fixed circuit constants may be arranged to automatically tuneto the frequency of energy supplied thereto within a desired working frequencyrange. In my copending application, Ser. N 0. 205,934, filed July 15,1927, since issued on July 15, 1930, as Patent No. 1,770,525, of which the present application is a continuation in part, I have illustrated the application of this principle of automatic tuning to areceiver consisting of a tuned for automatic tuning, and the primary object of. my present invention centers about the provision of an improved electron discharge, tube cascade-amplifying system of the so-called untuned type, or more accurately, having fixed circuit constants.

In my aforesaid Patent No. 1,673,287, I have disclosed how an electron emission tube may be arranged to be automatically tuned over a ran e of frequencies. For this purpose it is tfi 4o cuit of the tube works into a suitable succeed ing circuit, which preferably has the electrical characteristics of a small fixed loss-free capacitance. The output circuit of the tube is made naturally resonant to a frequency lower than the lowest frequency of the work ing frequency range. Frequencies in the working frequency range are thereforehigher, making the output circuit effectively capacitlvc, and therefore characterized by decreasing impedance with increasing freere assumed that the output cirquency, which in turn causes decreasing ra-' dio frequency potential variation on the anode of the tube, and therefore a decrease in the apparent input capacitance of the tube with increasing frequency, which may be utilized for automatic tuning by a proper selection of the input circuit connected to the tube.

If it is desired to have a plurality of automatically tuned stages arranged in cascade, instead of merely a single stage, an immediate difiicult-y arises because of the fact that the output circuit of any tube preceding the last tube, instead of consisting of a high inductance which remains resonant to a con-' stant frequency lower than the workin fre quency range, operates as an automatically tuned circuit and therefore tends to remain in resonance with the working frequency.

A primary object of my present invention is to overcome the foregoing ditficulty, and to make possible the use of a plurality of automatically tuned stages coupled directly in cascade. To this end I divide the working frequency range into a plurality of portions, and I arrange the automatically tuned stages to each tune over a different one of the frequency portions. I then couple the stages a plurality of stages coupled directly in cascade is obtainable, and at the same time no disadvantage is incurred in operating each stage over only a portion of the working frequency range, for the reason that with present commercial tubes, and without exceptional'precautions, it is anyway difiicult to obtain automatic tuning over all of so broad a range as the commercial broadcast range.

Volume control is desirable to vary the gain in an. amplifier, and is obtained by varying the steady or operating anode potential or the control electrode bias potential of the an lea either the anode or the control e plifier tubes. A change in either of these operating potentials changes theamount of anode current and otential variation resulting from a given input potential variation, and therefore changes the apparent capacitance of the input circuit of the tube. Also, a change in the operating potential applied to ectrode changes the anode to cathode tube impedance and thereforechanges the load across its output transformer or other circuit. These two changes affect the tunin of the circuits and the stability thereof. or example, if the anode potential is materially reduced the alternating anode potential for a given alternating rid potential is reduced, and the apparent input capacitance of. the tube is therefore greatly diminished. This raises the frequency of the circuit, so-that it may become inductive and therefore regenerative for some frequencies. At the same time the anode to cathode impedance of the tube is greatly increased, thereby effectively removing a resistance load across the output transformer.

This condition gives rise to an oscillatory or unstable state of the amplifier at some portion ofthe range of volume control.

The present invention, as a continuation of my copending application aforesaid, deals with these secondary problems which arise when it is attempted to apply volume control to an automatically tuned amplifier, and a desideratum of the present invention centers about the application of volume control means to such an am lifier, and to improvements in the ampli er which obviate instability when volume control is applied thereto.

In the automatically tuned stages the output circuits are naturally resonant to frequencies lower than the input circuits, and the stages normally are therefore stable.

. This relation may be lessened and even reversed at some point over a wide range of volume control,and accordingly one object of the present invention is to so adjust the input and output circuits of these stages, within the range of dimension giving the desired automatic tuning, that the natural resonance frequency of the input circuit remains higher than the natural resonance frequency of the output circuit over the full range of volume control.

In one of theamplifier stages, coming between automatically tuned stages, the input transformer is necessarily of greater inductance than the output transformer, and the latter is therefore naturally resonant to higher frequencies than the former, and therefore 1s inductive and regenerative 1n the working frequency range.

of the regenerative reaction in this stage, but variations in the constants of the circuits However, their wide difference 1n frequency lends stabilityf which tends to prevent oscillation in spite in the range of volume control may lessen this frequenc difference and again lead to instability. ccordingly, a further object of my invention is to overcome this possible source of instability, which I do by making the input and output circuits more widely different in frequency than would otherwise be necessary, and sufiiciently so to prevent instability over the range of volume control.

Variation in the steady or operating poten-. tials applied to thetubes I have found is disadvantageous when applied to the first or transition tubes of the amplifier. For ex-' ample, if the first tube is subjected to a variable operating potential the apparent input ca acitance into which the selector works is ma e variable, and this will throw the last tuned circuit of the selector out of tune relative to the preceding tuned circuits, and therefore lessen the selectivity thereof. If volume control is applied to the second transition tube a reduction in potential 'will' reduce theapparent input capacitance of the second tube and therefore change the capacitive load on the outputcircuit of the first tube, which in turn varies the reaction of the first tube, making it regenerative.

A further object of my present invention is to overcome'the foregoing difliculties, and to this end I find that I am able to prevent changes in the selectivity or/and tuning of a selector over the entire range of volume control by obtaining the desired volume control through variation of the operatin potential applied to the last or automatic y tuned group of amplifier tubes, but not to the transition stages.

To the accomplishmentof the foregoing and such other objects as will hereinafter appear, my invention consists in the methods .and the circuit elements and their relation diagram for a circuit e features of my inven- Fig. 5 is a dimensioned section through an insulation spool suitable for some of the bifilar astatic transformers employed in the amplifier;

Fig. 6 is adimensioned section through a spool for a bifilar transformer employed in the am lifier;

Fig. is a dimensioned section through a spool for a multi-section bifilar astatic'transformer employed in the amplifier; and

Fig. 8 is a wiring diagram for aslightly modified'amplifier similar to that shown in Fig; 2.

eferring now to Fig. 1 of the drawings,- I show a radio receiving apparatus comprising a selective system generally designated by the bracket S and an audion amplifying system generally designated by the bracket A, the selector systems comprising a plurality of circuits in cascade having variable constants to effect selectivity, and the amplifier A comprising cascaded electron discharge tube stages having fixed circuit constants, and being coupled to the lastselective circuit of. the selector S.

' The selector system- S is described in greater detailand claimed in my copending application, Ser. No. 205,935, filed July 15, 1927. while the inter-relation of the selector and the amplifier and the mode of preventing reaction by the latter upon the former is more particularly described and claimed in my copending application, Ser. N 0. 205,934,

'- also filed July 15, 1927, since issued as Patent No. 1,770,525, of which the present application is a continuation in part, the present application being directed to the amplifier, as there disclosed, and also to further improvements therein, here disclosed in connection with Figs. 2 through 8 of the drawing.

tive circuits S k 2 and S, preferably inductively coupled together. circuit S is coupled to an energy receptor ant by means of a condenser 10, and the last selector circuit S is coupled to the input circuit of the first'tube of the amplifier A.

In order to make this selective system S functionable in a radio receiving apparatus whereamplification is subsequently accomplished the selector system must be associated with an amplifier of proper characteristics, the amplifier being designed to prevent reactions upon the selector system which tend to destroy the selectivity thereof. Also, the amplifier should possess inherent stability,

and the amplifier as a whole should be prevented from picking up any energy except that fed thereto from the selector system, and

yet the amplifier must be sufiiciently powerful to increase the signal to a volume suitable for operating the audio system even thoughthe couplings in the selector system be madequite loose to realize full geometric selectivity.

The amplifier A comprises five tubes, T T T, T. and T Each of the tubes comprises the usualgrid, plate and filament g, pand respectively. the filaments of the tubes being connected by means of filament circuits to the A battery designated A+. The filement circuits may be suitably provided with he selector system comprises three selec The first selector 7 each of the tubes, a bypass condenser 21 being employed connecting the plus side of the B battery to the minus side of the A battery.-

The input and output circuits of "the tubes are connected to thetube electrodes and batteries as shown in the drawings.

The pair of tubes T and T are coupled together and to subsequent 'circuits of the series-in such a way as to eliminate or balance all of the reactions at the input terminals of the tube T throughout the wave length range of, the receiver. To balance the reactions at the input terminals of the first input circuit 2', the circuit network of the amplifying tubes T and T are so organized as to balance or neutralize reactions, so as to cause such network to act or behave as a preferably capacitive reactance which is pure, in order to avoid either damping or regeneration. To permit of uni-control for the selective system S, this amplifier circuit network should further be designed so as to produce a constant capacitance at the input terminals of the first input circuit 2''.

To attain this balance or neutralization of the reactions at the input terminals of' the first tube T, I first fix the constants of the output circuit 0 in accordance with the prin ciples described and claimed in my copending application Ser. No. 51,569 of August 21 1925, now Patent No. 1,713,132 of May 14, 1929, or in my Patent No. 1,620,661 of March 15, 1927, the natural frequency of this output circuit in connected circuit being greater than the highest frequency of the whole'frequency range through which the input circuit 71 is- 'tunable, so that there are produced feed-back reactions from the output circuit 0 to the input circuit z" over the whole frequency range of the receiver. For neutralizingthis feed: back reaction, I employ resistance means in the plate circuit of the tube T, which re-. sistance means is selected of such a magnitude as to cause a feed-forward action from the input to the output circuit equal to the feedback action produced by the output circuit through the capacitive coupling of the tube structure. This resistance is represented as R in Fig. 1 of the drawings, and is arranged preferably between the tube anode and the inductance of the output circuit. A

The output circuit 0 with this determined characteristic is then coupled to the input circuit by means of a coupling condenser K and an inductance M, the condenser K bethe band to be transmitted through the amplifier. Preferably also the plate side of the resistance R .is shunted by means of an inband and the output circuit 0 of the tube T ductance N and a condenser D. The induc-. is organized so as to be effective for productance M may be loosely coupled to the plate ing a rising input capacitance of the second inductance L, but preferably is closely coutube at the longer wave lengths for neutralizpled thereto. in" this residual feed-back reaction.

As-an example of suitable magnitudes of The tube T is also in this wayincidentalthese constants the section of resistance R ly arranged to partly provide automatic tunbetween the plate and the intermediate tap ing, in a manner which will be described in may be 920 ohms, the remainder of the refurther detail in connection with tubes T sistance R. about 707 ohms, the coupling conand T, but in the case of tube T the input denser K about 250 mmf.s, the inductance N circuit is made capacitive rather than reso- 22 microhenrys, and the condenser D 150 nant, witha view to obtaining acorrective efmmf.s. The inductance M is preferably difect upon tube T so as to make the amplifier vided into two sections, one section trans- A perfectly reactionless upon the selector S. former coupled to the inductance L and the In other words, the tube '1 provides some other a free section, the free section being gain but is primarily designed as a reactionabout 485 lni crohenrys and the transformer less link between the amplifier and the seleccoupled section about 150. microhenrys, both tor. The tube T provides substantialgain, arranged so as to inductively aid each other. and is partly automatically tuned, but its cir- The transformer coupled parts are prefercuit constants are primarily intended to furably bifilary wound. ther improve the reactionlessness of tube T With this'arrangement and with these The amplifier coupling tubes T and T constants there are residual variations in the described also possess an amplification charexact balance of the feed-forward and the acteristic which increases with the wave feed-back reactions over the wave length length. This characteristic is extremely derange due largely to feed-back reactions from sirable in connection with the cascadedca acthe tube T (distant stage feed-back as disitively tuned circuits preceding these tu es, tinguished from adjacent stage feed back). and especially when such circuits are used these variations being of the order of magni in connection with the loop type of receiving tude ofabout 5% of the reaction due to an adantenna. It has been found experimentally jacent stage feed-back. In accordance with that in radio receivers of this type there is my invention these residual variations in a considerable loss of sensitivity at the longthe balance of the feed-forward and feedor wave lengths as compared to that obtained back reactions are compensated for (so that at the shorter wave lengths. This loss of an exact balance between the feed-back and sensitivity is in part due to the lower recepthe feed-forward reactions are obtained) by tion eflicienoy of the loop at long waves, and producing variations in the input capacitance in part due to the less favorable ratio of L/C of the second tube T which may be done by which is obtained at the longer waves with predetermining the characteristics of its outcapacitively tuned circuits. Although each put circuit 0 This may be accomplished, tuned circuit showsarelatively small dropin first, by selecting the constants of the outamplification, of the order of 3 to 1, the ms put circuit 0' so as to just neutralize on the cading of these circuits causes a geometric inlong waves, this producing a residual overcrease in theratio which again compounds damping for the short Waves, and e n with the drooping characteristic of the loop by then neutralizing these residual damping so that where three tuned circuits are used in reactions by producing a varying inp cascade with the loop receptor, the relative acitance for the second tube T This input sensitivities a'ttheextremes of the wave length capacitance should rise or increase at the range e widely variable, The addition to longer wave lengths or, conversely stated, such a capacitively tuned system of a pairof should decrease at the shorter wave lengths untuned amplifier tubes (T and T having S0 s to neutralize the increasing damping a sharply rising sensitivity with wave length reactions at such shorter wave lengths. This te d 't o e t for th de rease in n itivit rising input capacitance with increasing and produces a receiver which responds much wave lengths I effect by selecting the output more uniformly to ignals t the various circuit 0 to have a natural frequency of the wavelengths within the range. I have found Order of and Preferably lower than the lowest that the non-tunable transformer 12 s having frequency of the wave length band. O h the resonant fre uency described will tend to wise stated, this means that the output circuit produce a sharp y rising amplification charo of the tube T is organized so that over acteristic desired. The rising amplification the wave length band there is produced an encharacteristic of this untuned transformer ergy feed-back reaction from the output cir is compounded with the rising amplification cuit to the input circuit through the gridcharacteristic due'to the circuit KM which plate capacitive couplingof the tube neutralcouples the output 0' and the input 2' this ized except for a residual feed-back reaccoupling apparatus and its function to protion at the long waves of the wave length duce the rising amplification being described and claimed in my'copending application Ser. No. 182,220 of April 9, 1927, now Patent 1,7 70,524 granted July 5, 1930.

I have further found that in order to secure the input capacitance characteristic desired the second or short wave mode of 0scil lat-ion of the transformer 72 8 must be either entirely eliminated or moved toa frequency range higher than the highest frequency to which the tunable input circuit may be adjusted. In addition to this, it is also desirable that this transformer have the highest possible efl iciency over the wave length range. I have discovered that it is possible to secure this combination of conditions by constructing the transformer bifilarly as described and claimed in my aforesaid application Ser. No. 195,631 to transformer and coil system since issued on October 22, 1929, as Patent No. 1,7 32,937 .Briefiy considered, this transformer is wound with a pair of wires twisted together with a pitch relatively large compared to the wire diameter. One of these wires forms the primary winding and the other the secondary winding, and similar ends of the two windings connect with the plate voltage supply and filamert connections of the tube. The bifilar winding is preferably random wound in a relatively narrow slot so as to form a coil of relatively small distributed capacity and of relatively high inductance; The inductance is made sufliciently great to give a natural frequency to the transformer in connected circuit as aforedescribed.

F llowing the coupling tubes T and T the amplifier A is madeto comprise a plu rality of so-called untuned audion amplifiers T Tand T followed by a detector tube 1366., which latter is connected to the audio amplifying system. While I have termed the tubes T and T untuned amplifiers, in accordance with their designation in the prior art, I construct-these tube stages in accordance with the principles described and claimed in my aforesaid Patent No. 1,67 3,287, the said amplifying stages function ng for producing what I prefer to call an automatic tuning effect, causing the production of high and efficient amplification over a considerable -or relatively wide wave length range. or band.

The tube T worksinto 'a detector tube the input circuit of which has the electrical characteristics of a small fixed loss free condenser. Transformer p 8 is made naturally resonant in circuit to a frequency lower than the lowest frequency of the working frequency range so that frequencies in the working frequency range are higher, and the transformer is effectively capacitive and charac terized by decreasing impedance with increasing frequency Which, in turn, "causes a decrease in the apparent input capacitance of the tube with increasing frequency. The

transformer p 8 is selected to resonate'with the in ut capacitance of tube T over the lower requencyportion of the working frequency range. For the higher frequency portion of the working frequency range the transformer p 8 is a relatively large inductance, and is resonant to a frequency lower than the higher frequency portion. The input circuit of tube T is effectively the output circuit of tube T because of the substantially unity coupling in a bifilar transformer. The said resonance frequency of this input circuit therefore in turn causes a decreasing apparent input capacitance I with increasing frequency in tube T 'for the higher frequency portion of the complete frequency range. Transformer p 8 is se- 'lected to resonate with the apparent input capacitance of tube T over the higher frequency portion of the range. In this manner the complete working frequency range is divided into a plurality of portions, and each of the stages T and T is automatically tuned over a different one of the frequency portions. The stages T and T are coupled directly together but in the order of decreasing frequencies, so that the transformer 79 s for example, while automatically tuned to a frequency in the lower frequency portion of the range, is not automatically tuned to a frequency in the higher frequency portion of the range, and is suitable for causing automatic tuning of transformer p 8 to said higher frequency portion of the-working frequency range.

It is of course possible to carry out the general theory of this arrangement by subdividing the working frequency range into as many portions as may be desired and employing a similar number of automatically tuned stages. However, inasmuch as the complete group of-automatically tuned stages consists in effect of only one highly efficient stage and a remainder of untuned and therefore less elficient stages it is obvious that each stage should most preferably be operated over as large a range of automatic tuning as isconveniently and efficiently possible, and to use .only so many automatically tuned stages coupled directly in cascade as may provenecessary tocover the desiredworking frequency range. 1

This automatic tuning I accomplish, more specifically, by coupling the tube structures T and T with a transformer p s having a natural wave length of about 318 meters, then coupling the tube structures T and T by means of a transformer p 8 having a natural wave length of about 343 meters, and coupling the tube structure T with the de-' tecting tube Det. by a transformer ps having a natural wave length of about 650 meters. Each of these three transformers 1s preferably'a bifilar transformer of the kind afore-describ'ed in connection with tram.

former p 8 the first being wound with about 210 turns, the second with 225 turns, and the third with 450 .turns. The input circuit of the detector is also preferably provided with a free section of induc tance 25, dimensioned in accordance with an invention which is further described and claimed in a copending application of Lester L. d ones and Jacob Yolles, Ser. No. 402,378'filed October 25, 1929. These various circuits are connected to the A and B battery as shown. The foregoing transformer constants assume the use of the 201A type of tube, in which the grid to plate capacitance is of the order of 8 mmf, the plate to cathode impedance is ofthe order of 8000 ohms, and tle amplification constant is of the order 0 8.

The principle underlying this arrangement of amplifying tubes is that the longer natural wave length of the last transformer of the series (12 8 has the eflect of producing a rising input capacitance change with increasing wavelength for the last amplifying tube T, which is suited or made to coincide with that desired characteristic of the input circuit of said tube T which would cause the said input circuit to vary its resonance frequency and thus to automatically tune itself over a considerable portion of the total wave length band. This same relation is carried out in the coupling of the tube T to the tube T the natural wave length of the transformer 32 8 being greater than the natural wave length of the transformer p 8 and causing tube T to be automatically tuned to the lower wave length portion of the total wave length band.

The inter relations between the circuits in series should also be considered. Thus the output circuit of tube T being tuned to 343 meters, is tuned to a wave length that this output would have if the next tube T had no relay action. However, when tube T has relay action, its tube input capacitance is higher than the geometric capacitance by an amount due to the reaction of the output circuit of tube T. This, causes a rise in the natural wave length of the 343 meter transformer, bringing it up to the region of 450 meters. The tube input capacitance curve of tube T is therefore not like the tube input capacitance curve of tube T because the output circuit of tube T,-instead of having a single resonant frequency, has a range of resonant frequencies determined by the automatic tuning efiect of tube T on the 343 meter transformer. The output circuit of tube T, which is designated as a 318 meter transformer, is a compromise for the capacitance characteristic of tube T*.' The output circuit of tube T by the arrangement shown is automatically tuned to be held resonant over a wave lengthband of 320 to 450 meters, while the output circuit of tube T is auto.-

awaser 343, and 650 meters refer to the transformers connected in circuit but withoutplacing the tubes in operation. When tube T is operated its apparent input capacitance is greater than the geometric capacitance, and this increases the wave length of the circuit between tubes T and T This increase in capacitance, of course, is a variable dependent upon the wave length of the energy being received'by the amplifier, and is of such a value that the wave length of transformer p 8 isincreased from 343 to around 450 meters when amplifying energy having a wave length of 450 meters. As the wave length of the received energy is increased the wave length of transformer 12 s is increased together therewith, and accurate automatic tuning, by which I mean coincidence of the received and resonant frequencies, may then be'obtained. For long wave lengths approaching 650 meters there is a slight departure from coincidence owing to the fact that the transformer pas is itself resonant to a wave length of only 650 meters, and not resonant to a frequency lower than the received frequency when the received energy is about 650 meters in wave length.

When the wave length of the received energy is decreased-below say 450 meters the input capacitance of tube T. falls off only very slightly, so that with a received wave length of say 300 meters the natural wave length of transformer 32 8 might be say 425 meters, and with a received wave length of 200 meters the natural wave length of transformer 12 8 might fall off to say 400 meters.

Transformer p 8 has a natural wave length connected in circuit and without relay action of about 318 meters. When tube T is given relay action its input capacitance is increased, but at low wave lengths the increase is very slight. For example, with received energy of 200 meter wave length the output transformer p s being resonant at say 400 meters, is far off resonance and constitutes a small output impedance, and therefore causes but little increase in the input capacitance of tube T bringing the .wave length of transformer P883 up to, say, 325 meters; When amplifying energy having a wave length of 300 meters the input capacitance of tube T is further increased, giving a natural wave length totransformer We of, say, 340 meters. From this point onthe recircuit thereof be resonant to a frequency lower than the received frequency.

The resonance curve of an automatically tuned stage is broad orfiat topped compared with that of an efficient tuned circuit, and there is a substantial range at either side of exact coincidence of frequencies where very excellent gain is obtained due to automatic tuning in spite of a departure from coincidence, and it is with reference to this broader range of automatic tuning or efiicient amplification that the wave length ran 'e of trans- ;former p s isconsidered to be rom 320 to 150 meters, and that of transformer p 8 is considered to begin as shortas 350 meters. The specified long wave len th for transformer p 8 of 550 meters is ar lower than the broad range of automatic tuning, and is even considerably lower than the range of exact coincidence, but -550 meters is specified'because the amplifier is assumed'not to be required for use with longer wave lengths.

Each stage does not cover exactly half of the range, the practice being to cover slightly more than half of the. range with the last stage. The size of the input circuit for the last amplifier tube is determined in order to obtainthe maximum amplification from that stage over as much of the frequency band as the input capacitance of the last tube makes-available for automatic tuning. This fixes theoutput transformer and therefore the input capacitance characteristic of the next to the last tube. By measuring and plotting the input capacitance characteristic. ofthe next to the last tube, a value of input inductance for this tube'may be selected which utilizes most efi ectively the variable portion of the thusly plotted curve for auto- 'matic tuning. This determines the range of the tube T while the balance of the band may be carried by the tube T Referring now to Fig. 2 of the drawing, there is a selector having variable circuit constants for tuning, followed. by an amplifier having fixed circuit constants, the arrangement being in general similar to that set forth in'Fig. 1 and in my copending application previously referred to, except that in this'casc these are four resonant circuits. It may further embody improvements set forth in a copending application of Lester L. Jones and Jacob Yolles, Ser. No. 403,160, filed Oc-' tober 29, 1929. 1

Energy collected on any suitable antenna circuit 2 is fed to a first resonant circuit, 4, which is loosely coupled to a second resonant circuit .6, which in turn is loosely coupled to a third resonant circuit 8 by means of a small auxiliary'coil 10, and the resonant circuit 8 is loosely coupled to a fourth and final resonant circuit 12. The various tuning circuits are loosely coupled in order to avoid double resonance peaks, and the circuits are suitably positioned or shielded so that non-adjacent tuning circuits are effectively decoupled.

The received energy of selected frequency is supplied to the inputcircuit of a tube T the output circuit of which includes a coil 16, which is inductive within the working frequency range and therefore causes a regenera .tive reaction. Resistance 18 is arranged in series with the inductance 16 in ordertov cause a feed forward of energy through the tube T which tends to neutralize the regenerative reaction of the inductance 16, and a condenser 20 is preferably connected in parallel with the resistance 18, all in accordance with the. disclosure of my copending application, Ser. No. 397 .632, filed October 5, 1929. This complex output circuit serves'to make the tube T approximately reactionless, so that its input characteristic is nearly that of a pure condenser of low fixed capacitance, this characteristic being most suitable as not affecting the tuning range, the damping, and the selectivity of the resonant circuit 12 of the selector.

The capacitance should be pure because the introduction of resistance in the tuned circuit will, if positive, broaden the tuning of the last circuit and necessitate closer coupling of the preceding circuits, thereby broadeningthrougha bifilar transformer 21, the primary of whichis connected in parallel with the complex output circuit previously described, and the secondary of which is connected across the control electrode to cathode circuit of the tube. Succeeding amplifier tubes T T and T are respectively connected in cascade through bifilar transformers 24, 28, and 32. The amplified output from tube T is supplied to a detector tube 4.0 through a bifilar transformer 36, in series with the secondary of which there is connected a free grid inductance 38. The radio a filter section 42, the audio frequency com ponent being supplied to an audio frequency amplifier or a reproducer through connections 1 to the terminalsmarked Output.

Considering the second stage including the tube T more in detail, the transformer 24 is selected to be naturally resonant when in circuit to a frequency lower than the lowest working frequency in order to obtain a rising apparent input ca acitance with decreasing frequency for t e tube T In the system of Fig. 2, the output circuit of the tube T is given a substantially higher impedance than would be correct for a reactionless stage, this being for the purpose of substantially boosting the amplification gain therein, as described more particularly in my copending application Ser. No. 397,632, filed October 5, 1929, previously referred to. As set forth in said copending application, I have found that both the inductive and-resistance components of the output circuit of the tube T may be each increased to a value approximately double that for producing a reactionless sta the impedance of this cir-v cuit being dou led in efiect, and this producing a substantially doubling in the gain for the tube T Doubling this impedance causes residual reaction through tube T and this reaction may be made regenerative over the low frequency end of the range by increasing the feed forward resistance 18 slightly less than proportionally to the increase of the inductance 16. The reaction will still be degenerative at the high frequency end of the rangebecause the slope of the reaction curve difiers from that desired. To correct for regenerative reaction capacitance may be hung onto the anode terminal of the output circuit of tube T The input circuit of tube '1 is therefore not correctly automatically tuned, but rather is made capacitive, which may be done by selecting a transformer 21 with a larger inductance than would be correct for automatic tuning, because for resonance the capacitance then should be decreased, and not being decreased, is in excess. But the efiective capacitance should decrease at the high frequencies and increase at the low frequencies, for more regeneration is needed at the high frequencies and more degeneration at the low frequencies, in order to change the slope of the reaction curve. This is accomplished by the variation in input capacitance proach to automatic tuning which is meanwhile obtained in the in ut circuit of tube T The desi of the circuits of tubes T and T is set orth in detail in m aforesaid copending application No. 39 ,632.

more properly the output circuit of tube T is capacitive, and therefore decreases in impedance with ncreasing frequencles which decreases the radio frequency potential on the anode of tube T and consequently causes a decrease in apparent input capacitance with increasing frequencies, as is desired for automatic tuning. For the available range of variation in capacitance the transformer 32 is selected to obtain automatic tuning of tube T over as large a portion of the range as is available beginning at the low frequency end of the range.

Transformer 32 is not suitable for automatic tuning of the input circuit of tube T over the same frequency portion because transformer 32 is kept in resonance to such frequencies. However, this transformeris related to frequencies higher than the portion to which transformer 32 is automatically tuned just as transformer 36 is related tothe lower frequency portion, that is to say, transformer 32 is naturally resonant to a frequency lower than the portion of the range of frequencies to which the input circuit of tube T is to be automatically tuned. Therefore, for frequencies lying in the higher frequency portion of the working frequency range, the output circuit of tube T is capacitive and therefore decreases in impedance with increasing frequencies' and so causes the decrease in the apparent input capacitance of tube T with increasing frequencies which is desired for automatic tuning. Transformer 28 is of small inductance, selected to resonate with the range of apparent input capacitance of tube T to the higher frequency .portion of the working frequency range.

Tube T difiers from the other amplifier tubes in having an input circuit of high inductance and an output circuit of low inductance. This tube is therefore not suitable for automatic tuning, but on the other hand, it causes an increase in amplification at the lower frequencies, and this is desirable in an amplifier receiving its energy through a selector which is capacitively tuned, like the selector here illustrated, because such a selector is'less eficient at the lower frequencies, and by making the amplifier more eflicient at the lower frequencies it is possibleto make the selector and am lifiepesworking together as a unit have a uni orm r pon\se.

' naturallyresonant to a low frequency, which tends to make the output circuit capacitive and degenerativein the working'frequency range, so that the stage is inherently stable.

Likewise, transformers 28, 32, and 36 have successively increasing inductance, and therefore are naturally resonant todistributed and successively decreasing frequencies,- so that the output circuits of tubes T and T are capacitive, thereby making their reaction degenerative, and making the stages inherently stable. In the case of tube T which is automatically tuned over only the higher frequency portion ofthe frequency range the output circuit including transformer 32 is inductiveand therefore regenerative in the lower frequency portion of the frequency range, but this stage is nevertheless inherently stable for the reason that the input circuit of tube T including the transformer '28 can never attain resonance and therefore cannot oscillate in the lower frequency portion of the frequency range. In the upper frequency portion of the frequency range where the ina put circuit of tube T can oscillate the output circuit is effectively capacitive and degenerative, thus preventing oscillation. Tube T unlike tubes'Tt and T has an input circuit of large inductance and an output circuit of small inductance, and therefore is regenerative and tends to instability, but oscillation is avoided by the wide frequency separation be-- tween the natural frequencies of transformers 24 and 28, in combination with the substantial load on the output circuit caused by the impedance of'the tubeT This load is placed effectively across all oscillatory input circuit elements of tube T byreason of the nearly unity coupling obtained in the bifilar transformer 24.

In accordance with further features of my invention volume control is applied to this amplifier. It is desirable that the volume con-' trol means be capable of varying the amount of amplification over wide limits. For example, ina five-stage untuned amplifier the control of volume from a whisper to full lou'd speaker volume for all stations, both near and far, requires a variation "of amplification ranging from full power or an amplification of 10,000 or more down to an amplification of less than one, i. e., an attenuative condition. Some methods of securin this variation of total gain are by varying t he' anode potential or by var ing the bias on the control electrodes of t 1e tubes. In either ofthese methods there occurs a condition in which, the tube impedance being greatly increased, the resistance load due to the anode to cathode resistance ofthe tube is reduced and eifective- 1y removed from the fixed transformers. At

- the same time the apparent input capacitance duced by a largeamount from the value it has at full amplification, which raises the natural frequency of the adjacent transformer, so that it may become inductive and regenerative in the working frequency range.- This combination of unloaded transformers and a variationsfrom normal automatic tuning in the dlrectlon of regeneration gives rise to an OSClllHtOIyOI' unstable conditlon 1n the amplifier. This defect is more sharply felt in i the case of volume control obtainedby variation of control electrode bias than in the case of variation of anode potential for the reason that the tube impedance changes more rapidly as a function of the amplification gain when the gain is varied by bias control than when varied by change of anode potential.

I have managed to modify the constants of the transformers in the range of automatic tuning so as to eliminate this instability without introducing any additional structure. ll find that the instability referredto will cause oscillation in the amplifier when the volume .control is varied to reduce the amplification to an intermediate value. The instability occurs at an intermediate rather than at lowest amplification for the reason that at very low or zero or negative gainsthe amplifier becomes inherently stable because of the ab- 'sence of that amplification which is obviously essential to all regenerative phenomena. The oscillation may occur of itself when the instability is great,but when it is of slight extent the oscillation will only occur when a strong radio signal is impressed on the amplifier, causing slight additional changes in the bias and anode voltage beyond that provided by the voltage supply sources.

I have-discovered that this instability is due to one output circuit attaining a higher natural frequency than a preceding input circuit, in combination with an input circuit virtually devoid of the normal damping exerted onit by the anode to cathode load of the preceding tube. More specifically, in the case of the stage including tube T in the present amplifier the natural frequency of the output circuit including transformer 32 may become higher than that of the input circuit including transformer 28 at reduced pacitances of tubes T and. T both diminish at reduced gain, that of tube T diminishes more rapidly than that of tube T. This effect carried sutuciently far willovercome the lower initial natural frequency of transformer 32 relativeto that of transformer 28, and to permit of a regenerative condition in tube T. 4 At the same time the increase in impedance of tube T at reduced gain reduces the-damping or resistance load thereof across transformer 28, thereby more readily permitting regeneration from transformer 32, to cause oscillation in transformer 28;

The cure for this condition of instability I find to hejtwo-fold. Firstly, in some'es to increase the inductance of the output trans former of a tube and associated pair of transformerscreating the disturbance so that under all cbnditions of volume control its natural frequency of vibration never incre to a higher value than that of the input circuit associated with the same tube. I

Secondly, in the third stage,- including ,tu T, where the input circuit has a lower natural freuency thanthe output circuit, the remedy is to slightly reduce the inductance of the output circuit and sli htly increase the inductance of the input clrcuit so as t9 separate by larger amounts than automatic tuning of the other stages requires the nat- .ural vibration or resonance frequency of the input and output circuitsto the point where the regeneration is insufiicient to cause oscillation in. the input.

In both cases the variations from the mean values determined by the requirements for automatic tuning are small, so that the loss of transformer 28 is slightly reduced, there by insuring that the input circuit of tube T will remain naturally resonant to a r eruency higher than that of the output circuit, so that the sta e including tube T will remain stable throng out the range of volume control.

Transformer 24 is slightly increased in in" ductance .thereby causing, together with the decreasein the inductance of tformer 28,

an augmented difference in the natural frequencies of the input and output circuitsof tube T, which leads to greater stability. The increase in inductance of transformer 24 causes the-natural frequency thereof to be lowered, and further separated from the frequency of the transformer 21,' and so prevents regenerative reaction through the tube T Transformer 36 might be increasedin inductance to increase the frequency difierence between transformers 32' and 36, but in the case of a circuit like the present one, wherein transformer 36 works into a detector tube, or more broadly, an apparent small loss free capacitance, this precaution is unnecessary for the reason that the low natural resonance frequenc of transformer 36 remains fixed and there ore transformer 36 can never make transformer 32 oscillate. These changes maybe illustrated with reference to Fig. 3 in which the amplifier tubes T T T, T, and T, and the detector tube automatic tuning.

by transformers 21, 24, 28, 8 Q11 heights of the rectangles rereeen natural wave lengths thereof. -lativ to the values determined in the abseuce'cfj Ydluine control, the transformer dimensions tered in accordance with the dotted line-5hr??? ings on Fig. 3, that is to say, the indu to. of transformer 28 is decreased, the incline j tance of transformer 32 may beleftfl 5% changed by reason of a reduction in the} ductance of transformer 28, while the inductance of transformer 24 is increased. should be appreciated that these changes all of second order magnitude, and are in 40 are schematically tended tocorrect what may be considered, s

only as secondary defects, and that therefore the changes are all within the range of dim'ension which will successfully give the desired Referring again to Fig. 2, it will be observed that the anode potential for tubes T .1

a r is sup lied directly through lead 52 while the V0 ume control means is arranged to affect the anode potential of only tubes T T and T connected through lead 5% to the variable resistance 50. This I find is important because if a variable potential is supplied to the anode of tube T the apparent input capacitance thereof is varied, and this varies the tuning of the resonant circuit 12 "and therefore alters the selectivity of'the seector.

If a variable potential is applied to the anode ,of tube T the apparent mput capacitance' thereof also is' varied, and this varies f the capacitance hanging on the output circuit of "tube T and thereby also varies the reaction of the amplifier upon the selector. Tn other words, the in ut capacitance characgo's teristic of the tube T having been selected to perfect the reactionlessness of tube T variation of this input capacitance by volume control injuries itscorrective efiect upon the tube T.

I therefore prefer to apply volume control only to tubes T T, and T and not to the transition or linlr' stages T and T How-. ever, it is possible .to carry the foregoing recautions for stabilizing the amplifier furt er and then to a ply volume control to all 0 the tubes, wit a slight sacrifice in selectivity. 1

Considering a few further details of the circuit in Fig. 2,it will be noted from the. :20 'wirin diagram that inductance 16 and trans ormers 21, 28,32, and 36 consist of son. tions connected in series and wound in o posits directions. These sections are su ciently spaced so that the desired total in- I25 ductance isobtained, but at the same time are made astatic with respect to external'magnetic fields, this arrangement being disclosed and described more in detail and claimed in my cop'ending application, Ser. No; 402,379,

filed October 25, 1929. Transformer 24 need not be made astatic, if desired, for the reason that it is resonant to a frequency considerably sections, for reasons next explained in connection with transformer 36, and such a multi-sectionconstruction would add to the manufacturing expense of the amplifier.

Transformer 36 is wound in four sections, whereas the other transformers are wound merely in two sections.- This is done to fulfill an. additional requirement other than astat-' icism, namely, that the transformer sections should not be naturally resonant to frequencies lying inside of the working frequency range, because this would tend to cause peaks or inflection points in the amplification and reaction characteristics which are undesired in the present amplifierf In accordance with the disclosure in my copending application,

former.

Ser. No. 402,37 9, filed October 25, 1929, previously referred to, this 'diificulty is overcome by subdividing each transformer into a number of sections such that the natural resonance frequency of a section lies outside of the working frequency range. In the case of transformers 21, 28, and 32, each section fulfills this requirement even though only two sections are employed. In the case of transformer 36, which has a relatively large inductance, it is necessary to obtain more than two sections if the natural resonance frequency of each section is to be removed from the working frequency band. It is convenient to use four equal sections because by winding alternate equal sections in opposite directions astaticism may be obtained and, in fact, the astaticism of a four section transformer such as the transformer 36, ismore perfect than that of a two section trans- This feature is not claimed herein, being. claimed in the copending-application Ser. No. 402,379, previously referred to.

It will further be noted that an inductance 38 is connected in series with the .secondary of transformer 36 and the control electrode of the detector tube 40. This inductance is selected of such magnitude as will series resonate with the apparent tube input capacitance of the detector tube at a frequency slightly higher than the highest frequency of the working frequency band, thereby tending to boost the amplification of the higher frequencies. This is of advantage whenever the energy output is supplied to a tube having a capacitive input reactance, as is the case with a detector tube (the output circuit of which is shunted forradio frequency energy) because with a capacitive input circuit and Jacob .Yolles,-Ser. No. 402,37 8, filed Getober 25, 1929. q

The following is asuitable set of quantitative design vvaluesand dimensions for the amplifier illustrated in Fig. 2. Resistance 18 may have avalue of 2800 ohms. Capaciv ductance 16 consists of two coils arranged tance 20 may be 24 micro-microfarads.

astatically, each coil consisting of 83 turns random wound in a pair of slots on a spool such -as is illustrated and dimensioned in Fig. 4, the outside diameter of the winding being about 9/16 of an inch, and the total-inductance of coil 16 being 0.22 milli-h. Trans-- former 21 is 'a bifilar transformer preferably formed by twisting primary and secondary wires together with a very long pitch and random winding the twisted wire in opposite directions in a pair of slots in order to obtain astaticism.' The transformer consists of 210 turns in each slot of a pair of slots on a spool arranged and dimensionedas is indicated in Fig. 5 of the drawing, the outside diameter of the winding'being about of an inch, and the transformer having an inductance of 1.16- milli-h. Transformer 24 is a bifilar transformer consisting of 600 turns random wound in a single slot on a spool such as is illustrated in Fig. 6 of the drawing, the outside diameter of the winding being about one inch, and the transformer having an inductance of 4.45 milli-h. Transformer 28 is a bifilar transformer consisting of 150turns wound in each of a pair of slotsturns wound in each of a pair of slots on a spool suchas is illustrated in Fig. 5 of the drawing, the winding having an outside diameter of 1; of an inch, and the transformer having an inductance of 1.16 milli-h. T ransformer 36 is a bifilar transformer consisting of 250 turns in each slot of 4 alternately oppositely wound slots on a spool such as is illustrated in Fig. 70f the drawing, the'outside diameter of the windings being of an inch, and the inductance of the transformer being 3.15 milli-h. Inductance 38 consists of 175 turns per slot in a pair of slots on a' spool such as is illustrated in Fig. 4 of the drawing,

the windings having an outside diameter of 11/16 of an inch, and the coil having an intance of say 3 micro-microfara rent heater t pe of tubes commercially denoted type .22 having a grid to 11m capacian e p fication constant of say 13 and a tube imped-v ance of sa 1300 ohms, l he spools are made this design resulting in a stable amplifier re-- gardless of whether the volume control is by variation of anode. potential or by variation of grid. bias, and regardless of whether the volumecontrol is applied to the last three tubes or to all five of the tubes.

Attention is next directed to Fi .18 of the drawing. This figureis substantial y identical with that shownin .Fig. 2 except that stage T is coupled to.' stage T by means of a; condenser 119 in series with a coil 121, in-

stead of by means of a bifilar transformer. The relative merits of the two coupling systems are discussed in my co-pending appli- V cation, Ser. No. 397,632, previously referred to. Fig. 8 also difiers in that the volume control resistor 150 is arranged to var the anode potential applied to all of the tu es instead of merely the potentialapplied to the lastthree tubes. 'In a very high grade receiver in which excellent selectivity is of greatimportance I believe it preferable not to emplo volume control'in the transition stages'l and T In this case the volume control may best be supplemented by the variable inductive coupling means for volume I control shown in Fig. 1 whereby the attenuation in the selector is made variable. This is to prevent the amplificationof very strong local v signals in T and T which would produce distortion in T when theB or G' potentials are greatly reduced. Fig. 2 with the selector coupling variation shown in'Fig. 1 therefor would represent my invention in preferred form, but I have illustrated volume control applied to all five tubes'in Fig. 8 because it is possible to produce a stable amplifier even when this is done. I

When the amplifier is arranged for volume v control by variation of anode potential in the manner illustrated in .the drawing the fol lowing design constants may be'emgloyed. Resistance 118 may be 2800'ohms. apacitance 120 may be 20 micro-microfarads. Inductance 116 consists'of 85 turns in each of .a pair of oppositel wound slots having a total inductance o 0. 23 milli-h. Capacitance 119 is 240 micro-microfarads. Inductance 121consists'of 160 turns in each of a pair of oppositely wound slots and has an inductance of 0.8 milli-h. Transformer 124 is a bi'filar wound transformer consisting of 550 turns wound in a single slot and having an inductance of 4.3 milli-h. Transformer a5 128 is a bifilar transformer consisting of 175 turns in each of a pa ir of oppositely wound slots and having an inductance of 0.81 milli-h. Transformer 132 is a bifilar transformer con- 'sisting of 210 turns ineach of a pair of oppos itely Wound slots fandhas an inductance of' alternating currenttubes of the 227 type is assumed. This amplifier is stable for anode potential variation o l-3 or 5 tubes.

If it is desired to use grid bias volume control instead 'of anode otential variation sta ility are necessary, transformer 124 be'- ing raised in inductance from 4.3 to 5.15

milli-hg and transformer 128 being decreased in inductance from 0.81 to. 0.63 milli-h.

' It will be apparent fi'om the foregoing description that I have provided a cascade vacuum tube amplifier having fixed circuit constants, and which is untuued in that sense but which nevertheless is automatically tuned to frequencies lying in a broad working frequency range. A plurality of tubes are automatically tuned, and the circuit constants are so adjusted that successive output circuits ,automatically tune preceding input circuitsi even though the successive stages are couple directly together in cascade. It will further be appreciated that the various dificulties met with when it is attempted to apply vol ume control to this amplifier circuit have also been solved, and that in consequence a stable and eflicient automatically tuned amplifier is made available, the ain in which may be varied overs very wi e range suitable for volume control of radio receiving circuits.

While I have shown and described my invention in the preferred forms, changes and modifications maybemade in the structures disclosed without 'de' arting' from the spirit of the invention, dc claims. Y

I claim: p 1. 111 automatic tuning with a plurality of amphfier stages having fixed constants, the method of making possible-automatic tuning while coupling successive stages directl in cascade which includes dividing the worli ing frequency range into a plurality of ortions and. adjusting the circuit constants 0 each of the stages. for automatictuning in different portions of the working frequency range.

. 2. In automatic tuning with a plurality ofhaving fixed constants, the

amplifier sta method of ng possible automatic tuning while coupling successive sta es directly in of the stages or automatic tuning in different portions of the worhng frequency range, ac

sliiht further changes in the direction of' ed in thefollowing no and cascading the automatically tuned stages in the order of decreasing frequency portions,

whereby the. input/circuit of a succeeding stage may operate as the output circuitfor automatically tuning a preceding stage.

3. In the adjustment of a cascade automatically tuned amplifier comprising electron emission tubes, coupling circuits arranged to automatically tune preceding coupling circuits, and volume control means, the method of preventing instability over the range, of

volume control'which includes so propor-' tioning the inputand output circuits of each a a er than the natural resonance frequency of the plifier having fixed circuit constants and vol-' output circuit over the range of volume control. 1

4. In the adjustment of a vacuum tube amume control means, the method of preventing instability over the range of volumecontrol in a stage of amplification made regenerative by reason of the output circuit beingnaturally resonant to a frequency higher than the input circuit which includes making the input and output circuits sufliciently. widely diiferent in frequency to prevent instability.

5. In the operation of a receiver comprising a sharply tuned selector, a pluralityof amplifier stages, a tube arranged as a transition stage between thea-mplifier stages and the selector, and volume-control means to vary the gain in the amplifier, the method of preventing changes in the operational characteristics of the selector over the range of volume control which includes varying the gain in the'amplifier stages but not varying the gain in the transition stage of the amplifier.

- 6. In the operation of areceiver compris- =ing a sharply tuned selector, a plurality of electron emission tube stages arranged. for

automatic tuning, a plurality of tubes arranged as transition stages'hetween the auto matically tunedstages and the-selector, and volume control means to vary the operating potential applied to the tubes in order to vary the gain therein, the method of preventing changes in the operational characteristics of the-selector over the range of volume control which includes varying the potential applied to the automatically tuned stages of the amplifier but not varying the potential applied a to the transition stages of the amplifier.

.7. A cascade automatically tuned amplifier comprising a plurality of electron emission tubes, fixed circuits coupling the tubes directly in cascade, each coupling circuit being naturally resonant to-a frequency lower 7 than'thatof the preceding coupling circiiit I in order to .obtain a decreasing capacitance characteristic with increasing frequency of for use over a relatively broad working frequency range comprisingv a plurality of electron emlssion tubes, transformer means coupling the tubes directly in cascade, the output transformer of each ofthe stages being naturally resonant to a frequency lower than that of the input transformer of each of the stages in order to obtain a decreasin capacitance characteristic with increasing frequency for automatically tuning each of the stages, and the constants ofeachof the stages being adjusted to be automatically tunable over'different portions of the working frequency range. c

10. A cascade automatically tuned amplifier' for use over a relatively broad working frequency range comprising a .plurality of electron emission tubes, transformer means coupling the tubes directly in cascade, the output transformer of each of the stages being naturall resonant to a frequency lower than that o the input transformer of each ofthe stages in orderto obtain a decreasing capacitance characteristic with increasing for automatically tuning each of frequency t e constants of each of the stages stages,

being adjusted to be automaticall tunable over different portions of the wor ing frethe constants of which are selected to be sub stantially reactionless, a second electron emission tube and associated circuits the constants of which are selected to correct for residual quency range, and the sta es being cascaded reaction in the 'first tube, and an additional grou of stages :each comprising a tube-and associated circuits the'constants' of. which are. selected to be automatically tunable over different portions of a' relatively broad working frequenc range, said stages being arran ed n casca e in the order of decreasing equenc'y.

- 12; A radio I fixed circuit constants comprising a first elecconstantsof which are selected to be subfrequency amplifier having stantially reactionless, a second electron emission tube and associated circuits the constants of which are selected to be automatitron emission tube and associated circuits the cally tunableand to correct for residual reaction inthe first tube, and an additional ferent portions of a relatively broad working frequency range, said stages being arranged in the order of decreasing frequency, and being coupled directly in cascade.

13. A radio frequency ampllfier havlng fixed circuit constants comprising a first electronemission tube, an output circuit therefor the constants of which are selected to make the tube substantially reactionless, a second electron emission tube, input and output c1rcuits therefor the constants of which are so related as to cause a rising apparent input capacitance characteristic with decreasing frequency in order to obtam automatlc tuning and to correct for residual reaction 1n the first tube, a third electron emission tube, and an additional group of stages each comprlsing a tube and associated circuits the constants of which are selected to be automatically tunable over different portions of a relatively broad working frequency ran e, sa1d stages being arranged in the order of careersing frequency, and being coupled directly in cascade.

14. A radio. frequency amplifier having fixed circuit constantscomprismg a first electron emission tube arranged to be substantially reactionless, 'a second electron emission tube, input and output transformers therefor respectively resonant to high and low frequencies in order to obtain automatic tuning and to correct for residual reaction in the first tube, an additional group of tubes,

and transformers therefor each naturally resonant to a frequency lower than that of a preceding transformer in order to obtain automatic tuning over successive portions of a relatively broad working frequency range.

' 15. A radio frequency am lifier having fixed circuit constants designe for operation over a relatively broad working frequency range, said amplifier comprising a first electron emission tube and associated circuits the constants of which are selected to be substantially reactionless, a second electron emission tube and associatedcircuits'the constants of which are adjusted for automatic tuning and to correct for residual'reaction in the first tube, a third electron emission tube, a fourth electron emissiontube having input and outlput circuits the constants ofwhich are so ree atedas to obtain automatic tuning over the upper portion of the working frequency range, and a fifth electron emission tube havmg input and output circuits the constants of which are so related as toobtain automatic tuning overthe lower rtion'o'f the working fre uency range, all c said tubes being conple directly in cascade.

=16, A radio frequency amplifier having fixed circuit constants designed for operation over a relatively broad working frequency range, said amplifier comprising a first electron emission tube, an output circuit there for the constants of which are selected to make the tube substantially. reactionless, a

' second electron emission tube, input and output circuits therefor the constants of which are so relatedas to cause a rising apparent input capacitance characteristic with decreasing frequency in order to correct for residual reaction in the first tube, a thirdelectron emission tube, a fourth electron emission tube having input and output circuits the constants of which are so related as to obtain automatic tuning over the upper portion of the working frequency range, and a fifth elec-I tron emission tube having input and output circuits the constants of which are so related as to obtain automatic tuning over the lower portion of the working frequency range, all

ofasaidtubes being coupled'directly in casca e.

17 An automatically tuned amplifier comprising anelectron emission tube, associated input and output circuits, and volume.control means, the constants of the input and output circuits of said tube being so proportioned that the natural frequency of the input circuit remains always higher than the natural frequency of the output-circuit over the whole range of volume control.

18. A cascade automatically tuned ampli fier comprising electron emission tubes, coupling circuits havin chosen as to automatically tune preceding coupling circuits, and volume control means, the constants-of the input and output circuits of a plurality of said tubes bein so proportioned, within the range provi g the desired automatic tuning, that the natural frefixed constants so Y quency of the input circuit remains always higher than the natural frequency of the output circuit control. q v

19. A cascade automatically tuned amplifier compelectron emission tubes, coupling circuits havin fixed constants so chosen as to automatically tune preceding coupling circuits, and volume control means over the whole range of volume to vary the operating potential apkplied to the erein, the

tubesm order to vary the gain constants 'of the in "ut "and output circuits of a lurality of sai tubes bein so proportione within the range provi mg the desired 1. Matic tuning, that the natural frequency of the input circuit remains always higherthan the natural frequency of the output circuit over the whole range of volume control, in order to;pr.event instability of the amplifier at any point in the range of volume control. p

20. A. cascade untuned amplifier compris ing a plurality of electron emission tube stages, coupling'means having constants recomprising electron emission tubes and cou:

pling circuits naturally resonant to difierent frequencies in or near the working frequency range, the output circuit of one of said tubes being naturally resonant to a fre-' quency higher than the input circuit of said tube and therefore regenerative, the natural frequencies of the input and output circuits of said tube being sufiiciently widely separated, in or near the working frequency range, to prevent instability.-

22.. An automatically tuned amplifier comprising electronemission tubes, coupling circuits naturally resonant to different frequencies in or near the working frequency range, and volume control means, the output circuit of one of said tubes being naturally;

resonant to a frequency higher than the input circuit of said tube, the natural frequencies of the input and'output circuits of said tube being suficiently widely separated. to

prevent-instability over'the range of volume control.

23. An automatically. tuned amplifier comprising electron emission tubes, coupling circuits naturally resonant to different frequencies, and volume control means for varying the operating potential applied to a plurality ofsaid tubes, said plurality of tubes having output circuits naturally resonant to frequencies lower thanthe input circuits, one of said'tubes havin an output circuit naturally resonant to a eq'uency higher than the input circuit of said tube, the natural frequencies of the input and out ut circuits of said, tube being sufiiciently wi ely separated to prevent instability over the range of volumecontrol. v

24. An untuned amplifier comprising -a preceding circuit, a group of automatically. tuned stages, a stage between said receding circuit and said group of stages, t e output circuit of said stage having a natural frequency higher than that of the input circuit, and volume control means for varying the total gain over a wide range, the natural frequency of the circuits of, said stage being widely separated in order to prevent instability over the range of volume control.

fee

25. An untuned amplifier comprising a preceding circuit, 'a roup of automatically tuned stages, a stage etween said preceding circuit and said grou of stages, the outputcircuit of; said stage quency higher than that of the input circuit,

aving a natural. freand volume controlmeans fdr varying the .total gain over a wide range, the natural frequency of the circuits ofsaid stage being widely separated in order to prevent instability over the range of volume control, and the input and output- -circuit s. of said automatically tuned stages being so proportioned that the natural frequency of the input circuit remains always higher than the natural frequency of the output circuit over the range of volume control.

26. The combination with a radio circuit comprising a sharply tuned selector, amplifier stages, and a transition stage between the selector and the amplifier stages, of

means to control the gain in the amplifier stages, but notin the transition stage.

27. The combination with a radio circuit comprising a sharply tuned selector, automatically tuned amplifier stages, and a transition amplifier sta e between the selector and. the automatica y tuned stages for preventing selectivity disturbing reactions from the automatically tuned sta es to the selector, of means to control t e gain in the amplifier comprising means to vary the operating potential applied to the automatically tuned stages, but not that applied to the transition stage.

28. The combination with a radio circuit comprising a sharply tuned selector automatically tuned amplifier stages, and transition amplifier stages between the selector and the automatically tuned stages, or means to control the gain in the amplifier comprising means to vary the operating potential applied to the automatically tuned stages,

but not thev operating potential applied to the transition stages, in order toprevent variations in the operational characteristics of the selector.

29. The combination of a-selector system com rising a pluralit of cascaded circuits tuna le over a broad and'and' having'overall sensitivity decreasing with wave length over the tunin ran e, a, non-tunable amplifier connecte to t e selector s stem, and

means in said non-tunable ampli er for amplifying the said band with a gain characteristic increasing with wave length over the tuning range so as to compensate for the decreasing sensitivity of the selector system. 30. The combination of a selector system comprising a plurality of capacitively tuned cascaded circuits tunable over a broad band and having an overall sensitivity decreasing with wave length over the tumng range,

non-tunable tube amplifier system connected to the selector system, and means in said amplifier for amplifying this band with a gain characteristic increasing with-wave length over the tuning range, whereby the combination possesses a high and reasonably con,- stant degree of amplification. i

31. The combination of a, selector syste tuliable over it broad band and havingsensitivity decreasing with wavelength over the tuning range, a non-tunable amplifier connected to the selector system for amplifying this band with a gain characteristic increasingwith wave length over the tuning range, and means for'eliminatin the selectivity disturbing reactions of sai emplifier on said selector system.

Signed at New York in the county of New 0 York and. State of New York this 2nd day of October, A. D. 1929.

, LESTER L. JONES. 

